The movement and development of cells within the reproductive systems of Schistosoma mansoni and Schistosoma japonicum was traced by labeling with H3-thymidine, transplanting to uninfected hamsters for various periods, and autoradiography of the recovered worms. Males of both species showed labeled sperm in the seminal vesicle 6 days after labeling. Labeled sperm were detected in the seminal receptacles of females on day 6. Females of both species showed a dilution of label in nuclei of developing oogonial cells within 3 days. However, label was never detected in nuclei of primary oocytes even after periods of 18 days. Label in nuclei of vitelline cells becomes dilute by day 1 and is undetectable in the cells of vitellaria at day 5. A few heavily labeled cells were observed in the vitelline duct after day 1 and on day 2 and 3 became progressively more numerous and less heavily labeled. Adults of S. japonicum transplanted to the peritoneal cavity of hamsters were still alive at 6 days but exhibited degenerating reproductive organs. In vitro culture of S. mansoni using a medium designed by Michaels and Prata showed dilution of label in testes, ovary, and vitellaria, but these organs exhibited signs of necrosis after day 5. This medium was even more unsuitable for S. japonicum because adults were dead after day 5. Unisexual infections of male S. mansoni showed no deleterious affects on sperm production. Female S. masononi in unisexual infections exhibited degenerative changes in the reproductive system as early as 1 day after transplantation.